This invention relates to elastomeric polyurethane and/or polyurethane/urea products and more particularly to a unique composition consisting of a prepolymer and a dibasic ester additive which reduces viscosity and improves wettability of the castable polyurethane prepolymer. This invention relates further to an effective process for improving wettability of the liquid prepolymer to polyester fabric during the manufacture of a polyurethane coated fabric type of belting.
The additive of this invention imparts the improvements above without reduction in hardness and eliminates the deleterious effects that occur with other additives classed as plasticizer or diluents. At the preferred ratios of the dibasic acid ester with the aromatic diisocyanate prepolymer, this invention has in fact been found to increase certain properties, modulus, tensile strength, or tear strength. The claimed composition does not reduce other important cured properties at levels of addition that other plasticizer additives do exhibit degraded physical properties.
Aromatic polyisocyanates are well known and are widely used in the preparation of polyurethane and poly urethane/urea elastomers. These aromatic diisocyanates generally include compositions such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4'-methylene bis (phenylisocyanate) and the like. In the preparation of polyurethane and polyurethane/urea elastomers, the aromatic diisocyanates are reacted with a long chain (high molecular weight) polyol to produce a prepolymer containing free isocyanate groups which then may be chain extended with a short chain (low molecular weight) polyol or aromatic diamine to form a polyurethane or polyurethane/urea elastomer. Long chain, high molecular weight polyols, e.g. those having a molecular weight of above 250, are generally utilized to form the prepolymer and the chain extender is generally a short chain polyol, e.g., C.sub.2 -C.sub.10 polyol, or an aromatic diamine. The long chain, high molecular weight polyol provides flexibility and elastomeric properties to the resin, while the short chain polyol or aromatic diamine provides chain extension or cross-links and adds toughness and rigidity to the resulting elastomeric polymer.
It is known that certain additives termed plasticizer or diluents when added to polyurethane prepolymers will reduce the viscosity of these prepolymers. A simultaneous effect also with these additives is to significantly reduce certain cured properties of the polyurethane elastomer such as hardness, modulus and tensile strength. Surprisingly, it has been found that by incorporating DBE (dibasic ester) in the prepolymer, the desired properties of improved fabric wetting time, and reduced viscosity are achieved without reducing the hardness, modulus, tensile strength and tear of the cured elastomer. In certain ratios of DBE to prepolymer, critical properties of modulus, tensile strength and tear strength are actually found to improve over the same prepolymer without DBE. This improvement in properties is very surprising and has heretofore not been possible when known plasticizers are added to urethane prepolymers.